Reactor and plant for the continuous preparation of high-purity silicon tetrachloride or high-purity germanium tetrachloride

ABSTRACT

A reactor and a plant containing the reactor for conducting a continuous, industrial process for preparing high-purity silicon tetrachloride or high-purity germanium tetrachloride is provided. The plant contains a plasma reactor having a dielectric, a high voltage electrode and an earthed, metallic heat exchanger, in which the longitudinal axes of the dielectric, of the high-voltage electrode and of the earthed, metallic heat exchanger are oriented parallel to one another and at the same time parallel to the force vector of gravity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior U.S. applicationSer. No. 12/065,126, filed Feb. 28, 2008, the disclosure of which isincorporated herein by reference in its entirety. The parent applicationis the National Stage of PCT/EP06/62916, filed Jun. 6, 2006, thedisclosure of which is incorporated herein by reference in its entirety.The parent application claims priority to German Application No.102005041137.1, filed Aug. 30, 2005, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a continuous process, a plant and areactor for the preparation of high-purity silicon tetrachloride orhigh-purity germanium tetrachloride by treatment of the silicontetrachloride or germanium tetrachloride to be purified, which iscontaminated with at least one hydrogen-containing compound, by means ofa cold plasma and subsequent fractional distillation of the treatedphase.

Silicon tetrachloride (SiCl₄) and germanium tetrachloride (GeCl₄) areused, inter alia, for producing optical fibres. For these applications,SiCl₄ having a very high purity is required. Efforts are likewise madeto prepare germanium tetrachloride in very pure, in particularhigh-purity, form.

Here, hydrogen-containing compounds are a considerable disadvantage,even if they are present in only ppm amounts. However, silicontetrachloride frequently contains hydrogen-containing trace componentsor secondary components, e.g. HCl, —Si—OH-containing species,—C—H-containing species and Si—H-containing species. An analogoussituation applies to GeCl₄.

In the case of hydrogen-containing impurities in silicon tetrachloride,a distinction has to be made between impurities which are difficult toseparate off and those which are easy to separate off. HCl, for example,can be separated from silicon tetrachloride down to the region of <1 ppmby weight by simple fractional distillation. On the other hand,hydrocarbons in particular but also chlorinated hydrocarbons andpossibly corresponding compounds such as silanes bearing alkyl groupscannot be separated off down to the region of <1 ppm by weight by simplefractional distillation.

Possible ways of removing hydrocarbons, chlorinated hydrocarbons andcorresponding compounds such as silanes bearing alkyl groups fromsilicon tetrachloride have been known for a long time.

Thus, silicon tetrachloride containing the abovementioned componentscan, according to U.S. Pat. No. 4,372,834 and EP 0 488 765 A1, betreated in the presence of chlorine with UV radiation in the wavelengthrange from 200 to 380 nm and the chlorination products obtained cansubsequently be separated from SiCl₄ by fine distillation. A substantialdisadvantage of this process is that the plant components come intocontact with chlorine gas, which is added in considerable amountsaccording to EP 0 488 765 A1, and are thus subjected to particularlysevere corrosion, which inevitably leads to frequent shutdowns of theplant. In addition, the chlorine to be added likewise has to meet veryhigh purity requirements. Both result in high operating costs for theplant. A further particular disadvantage is the particularly poor energyefficiency of UV radiation sources proposed, for example, by EP 0 488765 A1. This results in particularly long treatment times, whichlikewise leads to high costs.

A general process for purifying halogen compounds and hydrogen compoundsof silicon is likewise known (DE-B 10 58 482). Thus, chlorosilanes andbromosilanes can be treated by addition of a reducing agent such ashydrogen, silicon, sodium, aluminum or zinc under the action of a gasdischarge, in particular a dark gas discharge, forming, as a result offree radical formation and combination of free radicals, relatively highmolecular weight compounds in which the elements carbon, boron orphosphorus can be incorporated in relatively high molecular weightcompounds of chlorosilicon and which are separated off by distillation.A particular disadvantage of this process is the fact that a reducingagent has to be added. In particular, DE-B 10 58 482 teaches theaddition of hydrogen as reducing agent in the purification of an SiCl₄fraction.

Plasma technology has a special place in the generation of ozone fromoxygen or air in an ozonizer (EP 0560 166 A1, WO 89/12021, WO 97/09268,WO 01/07360, WO 02/096798, WO 04/046028).

The earlier German patent application 10 2004 037 675.1 teaches acontinuous process for the preparation of high-purity silicontetrachloride or high-purity germanium tetrachloride by treatment ofsilicon tetrachloride or germanium tetrachloride contaminated with atleast one hydrogen-containing compound by means of a cold plasma andsubsequent fractional distillation of the phase which has been treatedin this way. A cold plasma can in principle also be generated usingozonizer systems. However, reactor systems for producing high-puritysilicon tetrachloride or germanium tetrachloride generally require theuse of spacers for fixing a precise distance between the electrodes andthe dielectric. Furthermore, suitable spacers are only moderatelyresistant to SiCl₄ or GeCl₄.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a further possibleway of carrying out a continuous process for the preparation ofhigh-purity silicon tetrachloride or high-purity germanium tetrachlorideby treatment of the silicon tetrachloride or germanium tetrachloride tobe purified by means of a cold plasma on an industrial scale.

In the following, silicon tetrachloride or germanium tetrachloride willalso be referred to as tetrahalides for short.

The nominated object is achieved according to the invention as describedin the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a plant according to one embodimentof the invention.

FIG. 2 shows a micro unit of a plasma reactor for gas phase treatmentaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has surprisingly been found that the continuous process for thepreparation of high-purity silicon tetrachloride or high-puritygermanium tetrachloride by treatment of silicon tetrachloride orgermanium tetrachloride which is contaminated by at least onehydrogen-containing compound with a cold plasma can be carried outadvantageously and reliably on an industrial scale in a comparativelysimple and economical manner using a novel reactor system, a plasmareactor for gas-phase treatment (PRG for short) and subsequentfractional distillation of the phase which has been treated in this way,with such a reactor advantageously having the dielectrics present in thePRG arranged in a mechanically self-stabilizing fashion, which isachieved by parallel arrangement of the generally tubular dielectricsrelative to one another and direction of their longitudinal axesparallel to the force of gravity.

In addition, a high aspect ratio (gap for short), i.e. aspect ratio=tubelength/charge spacing, with an extremely high homogeneity and thus ahigh constancy of the gap over the length of the dielectric (“tubelength”) can advantageously be achieved. In this way, the spacerscustomary for ozonizers can be omitted in an advantageous fashion on thebasis of the teachings of the invention.

Furthermore, the phase to be treated (A) can appropriately be fed intothe plasma or reaction region in the present PRG at the earth end, whichcontributes to stabilization of the reactor arrangement, cf. FIG. 2.

Thus, a plant of the present type makes it possible for industrialtreatment and purification of silicon tetrachloride or germaniumtetrachloride phases by means of a cold plasma to be carried out in asurprisingly simple and economical way. In addition, such an industrialplant can be operated safely and reliably using the present process. Tomonitor the effectiveness of the present process, methyltrichlorosilane(MTCS), in particular, can be used as guide parameter.

Thus, an SiCl₄ grade according to the invention advantageously containsless than 1 ppm by weight of MTCS, with the analytical detection limitof MTCS in SiCl₄ being 0.2 ppm by weight. The determination of MTCS canbe carried out by means of FT-IR or 1H-NMR methods.

Furthermore, it has surprisingly been found that the procedure describedabove for silicon tetrachloride can also be used for the purification ofgermanium tetrachloride.

FIGS. 1 and 2 show, by way of example, a preferred embodiment of a plantaccording to the invention or a PRG and also a micro unit of a PRGaccording to the invention.

FIG. 1 shows the flow diagram of a preferred plant for carrying out theprocess of the invention:

-   A starting material to be treated-   B hydrogen chloride gas-   C low boilers-   D high boilers-   E product stream-   1 storage vessel (heatable, coolable)-   2 transport unit-   3 vaporizer-   4 plasma reactor for gas-phase treatment (PRG)-   5 condenser-   6 transport unit-   7 intermediate vessel-   8 transport unit-   9 distillation column (heatable)-   10 low boiler removal-   11 transport unit-   12 distillation column for separating off high boilers-   13 condenser-   14 transport unit-   15 product container

The present invention accordingly provides a reactor for the treatmentof silicon tetrachloride or germanium tetrachloride which iscontaminated by at least one hydrogen-containing compound by means of acold plasma, wherein the plasma reactor (4) is based on a reactorhousing, a high-voltage supply and at least one micro unit for theplasma treatment which consists essentially of an earthed, metallic heatexchanger (4.2), a dielectric (4.4), a perforated plate, a grid or amesh (4.1) and a high-voltage electrode (4.3) and the longitudinal axesof the dielectric (4.4), of the high-voltage electrode (4.3) and of theearthed, metallic heat exchanger (4.2) are oriented parallel to oneanother and at the same time parallel to the direction vector of theforce of gravity.

The present invention further provides a plant for the preparation ofhigh-purity silicon tetrachloride or high-purity germanium tetrachloridein a reactor system (1, 2, 3, 4, 5, 6, 7, 8) by means of a cold plasmaand a subsequent distillation unit (9, 10, 11, 12, 13, 14, 15) for thework-up of the treated phase, wherein the plasma reactor (4) (PRG forshort) is based on a reactor housing, a high-voltage supply and at leastone micro unit for the plasma treatment which consists essentially of anearthed, metallic heat exchanger (4.2), a dielectric (4.4), a perforatedplate, a grid or a mesh (4.1) and a high-voltage electrode (4.3) and thelongitudinal axes of the dielectric (4.4), of the high-voltage electrode(4.3) and of the earthed, metallic heat exchanger (4.2) are orientedparallel to one another and at the same time parallel to the directionvector of the force of gravity.

Preference is here given to a tubular dielectric (4.4) being oriented ona perforated plate, a grid or a mesh (4.1), with the dielectric (4.4)being surrounded by a tubular heat exchanger (4.2) and a rod electrodeor a tubular mesh electrode (4.3) projecting completely or partly intothe tube (4.4), cf. FIG. 2.

Furthermore, preference is given to the dielectric (4.4) being a tubehaving a wall thickness from 0,1 to 10 mm, preferably from 0.5 to 2 mm,in particular about 1 mm, an internal diameter from 1 to 300 mm,preferably from 5 to 30 mm, in particular about 10 mm, and a length from10 to 3 000 mm, preferably from 500 to 2 000 mm, in particular from 1000 to 1 500 mm, and comprising quartz glass, Duran glass, borosilicateglass or aluminum oxide. In addition, the surface of the dielectric canbe structured, in particular in order to maximize the geometric surfacearea of the dielectric.

In a PRG according to the invention, the electrode (4.3) isadvantageously made of metal or a metal alloy and may be coolable.

Preference is also given to the respective shortest open spacing (d)between a dielectric (4.4) and the high-voltage electrode (4.3) andbetween the dielectric (4.4) and the tubular heat exchanger (4.2) beingidentical, cf. FIG. 2.

Particular preference is given to the spacing (d) being from 0.01 to 100mm, particularly preferably from 0.1 to 10 mm, very particularlypreferably from 0.3 to 1 mm.

In a plant according to the invention, the perforated plate, the grid orthe mesh (4.1) in the PRG appropriately have a free cross-sectional areaof from 10 to 90%, preferably from 20 to 80%, particularly preferablyfrom 30 to 70%, very particularly preferably from 40 to 60%.

Furthermore, the heat exchanger (4.2) of the PRG can be both heatableand coolable and be configured as a shell-and-tube heat exchanger.

A plant according to the invention is preferably based on at least onePRG (4) which advantageously comprises from 2 to 50 000 micro units,particularly preferably from 20 to 10 000, very particularly preferablyfrom 600 to 6 000, in particular from 1 000 to 4 000, micro units.

The present invention likewise provides a continuous, industrial processfor preparing high-purity silicon tetrachloride or high-purity germaniumtetrachloride by treating the silicon tetrachloride or germaniumtetrachloride to be purified, which is contaminated by at least onehydrogen-containing compound, by means of a cold plasma and isolatingpurified silicon tetrachloride or germanium tetrachloride from theresulting treated phase by fractional distillation, wherein thetreatment is carried out in a plasma reactor (4) in which thelongitudinal axes of the dielectric (4.4), of the high-voltage electrode(4.3) and of the earthed, metallic heat exchanger (4.2) are orientedparallel to one another and at the same time parallel to the forcevector of gravity.

According to the invention, preference is given to at least one microunit per PRG (4) whose discharge space is preferably tubular and free ofstands, with the longitudinal axis of a tube being oriented essentiallyparallel to the force vector of gravity, i.e. perpendicular to theearth's surface.

Preference is thus given in the process of the invention to the use ofat least one plasma reactor for gas-phase treatment (4) whose tubulardielectrics (4.4) stand on a perforated plate, a grid or a mesh (4.1),with the phase to be treated (A) flowing through the perforated standingarea (4.1) and subsequently passing through the reaction region betweenthe dielectric (4.4) and the electrode (4.3 or 4.2).

However, two or more plasma reactors (4) can also be operated in seriesor in parallel.

Such tubes (4.4) of a preferred PRG (4) are generally based on quartzglass, Duran glass, borosilicate glass or aluminum oxide. A preferredPRG is usually operated using a pulsed alternating current.

Reactors (4) used in the process of the invention are preferablyequipped with from 1 to 50 000 micro units which are orientedperpendicular to the earth's surface and are arranged parallel to oneanother.

The process of the invention is advantageously carried out using a coldplasma in the form of a dielectrically hindered discharge (DHD or silentdischarge) which is advantageously generated in each micro unit of thePRG. The present invention thus also has the advantage that the presentprocess can be carried out as microtechnology in microreactors, forexample consisting of one micro unit, with a plurality of such microreactors being able to be operated in parallel and/or in series.

Alternating current discharges having frequencies in the range from 1 to10⁹ Hz, in particular from 10 to 10⁶ Hz, are preferred for the treatmentof the abovementioned tetrahalides in the process of the invention.Here, pulsed barrier discharges or barrier discharges operated using anAC voltage are preferably used.

A barrier discharge can be generated between two metallic electrodes ofwhich at least one is covered with a dielectric which prevents sparkformation or arcing between the two metallic electrodes. Instead, manybrief and spatially tightly limited microdischarges whose discharge timeand quantity of energy are limited by the dielectric are formed.Suitable dielectrics are ceramics, glass, porcelain or insulatingpolymers, for example Teflon. Further suitable materials are described,for example, in VDE 0303 and DIN 40 685.

Barrier discharges can appropriately be operated at pressures of from0.1 mbar to 10 bar. The electrical excitation of the discharge iseffected by applying an alterable voltage to the electrodes. Dependingon the pressure in the discharge space, the spacing of the electrodes,frequency and amplitude of the AC voltage, discharges which last only afew nanoseconds and are randomly distributed in space and over time areformed when an ignition voltage is exceeded.

The electrical excitation can be characterized as follows:

Application of an AC voltage to the two electrodes results in ignitionof the desired discharge when the field strength in the discharge volumeis high enough. The voltage required depends on the free spacing (d)between the dielectric and the counterelectrode, on the dielectric usedand on the pressure in the discharge section, on the gas composition andon any internals present between the dielectrics in the discharge space(gap for short). The spacing (d) is appropriately set to from 0.01 to100 mm, preferably from 0.1 to 10 mm, in particular from 0.3 to 1 mm.The voltages required can be from 10 V to 100 kV, preferably from 100 Vto 15 kV, particularly preferably from 1 kV to 10 kV, in a micro systemor a micro unit. The frequency of the AC voltage is advantageously inthe range from 1 Hz to 30 GHz, preferably from 50 Hz to 250 MHz, inparticular from 600 Hz to 2 kHz. Further emitter frequencies areexplicitly not ruled out.

The PRG configured according to the invention can, however, also befilled with spheres or pellets for carrying out the present process. Theelectric discharge takes place first and foremost in the form ofcreeping discharges on the surface of the spheres or pellets, whichpreferably leads to an increase in the discharge surface. As a result,the concentration of ions and free radicals in the spatial vicinity ofthe surface produced in this way is increased and thus contributes toincreased reaction of the hydrogen-containing compounds present in thegas stream. In addition, such spheres or pellets can advantageously leadto a further improvement of the flow or mixing conditions, i.e. lead toa very uniform gas distribution in the discharge or reaction region.

Spheres or pellets used here can advantageously comprise a supportmaterial selected from the group consisting of aluminum oxide, titaniumoxide, zirconium oxide, cerium oxide, silicon dioxide, magnesium oxideand mixed oxides thereof. Preference is given to silicon oxide pellets(glass pellets).

When spheres or pellets are referred to below, this includes particles,powders or pulverulent substances or other particle size states. Thediameters can vary in the range from 100 nm to 10 mm, preferably from 10μm to 0.5 mm.

The electrodes of the plasma reactor can be configured as flatstructures aligned parallel to one another or can form a coaxialarrangement with a central electrode which is surrounded by a tubularelectrode and is preferably configured as a shell-and-tube heatexchanger. To aid the formation of discharges, spatial inhomogeneitiescan be provided, for example by means of helical electrodes which leadto large local field increases and thus to improved formation of thedischarge (ignition).

In the case of the “discharge hindered on one side”, it is possible, asindicated above, for one wall to consist of an electrically insulatingmaterial, e.g. fused silica or oxide ceramic, and one reactor wall toconsist of an electrically conductive material, e.g. stainless steel. Inthe case of the “discharge hindered on two sides”, both walls generallyconsist of electrically insulating material (dielectric having a highbreakdown voltage). Here, the electrodes should then be provided for theintroduction of, for example, the electric energy provided by means of apulsed DC voltage source.

Furthermore, one or more reactors can be used for generating the gasdischarge for the treatment of the tetrahalide to be purified in theprocess of the invention. If more than one reactor is used, the reactorscan be connected in series or in parallel.

As is known per se, the electron energy introduced in a plasma dischargeis dependent on the product of pressure p and electrode spacing d (p·d),so that at constant gas pressure particular free-radical reactions canbe promoted or suppressed in the plasma simply by means of a change inthe geometry of the reactor. In the process of the invention, theproduct of electrode spacing and pressure should be in the range from0.01 to 300 mm·bar, preferably from 0.05 to 100 mm·bar, particularlypreferably from 0.08 to 0.3 mm·bar, in particular from 0.1 to 0.2mm·bar.

The discharge can be excited by means of various AC voltages or pulsedvoltages of from 1 to 10⁶ V. Furthermore, the shape of the curve of thevoltage applied for generating the discharge can, for example but notexclusively, be rectangular, trapezoidal, sinusoidal, triangular, pulsedor made up of pieces of individual voltage-time curves. Furthermore, theproduction of suitable voltage-time curves can also be effected byFourier synthesis.

To achieve a high electron density and a very uniform formation of thedischarge in the entire discharge space of the reactor, pulse-shapedexcitation voltages are particularly useful. The pulse duration inpulsed operation depends on the gas system and is preferably in therange from 10 ns to 1 ms. The voltage amplitudes can be from 10 V to 100kV, preferably from 100 V to 10 kV, in a micro system. These pulsed DCvoltages can be operated and modulated at high repetition rates, forexample from 10 MHz in the case of the 10 ns pulse (pulse dutyfactor=10:1) to low frequencies (10 to 0.01 Hz), for example as “burstfunctions” to allow the reaction of adsorbed species.

The PRG used in the process of the invention can be made of anyelectrically and thermally suitable material. Stainless steel incombination with plastics, ceramics and glasses is particularlypreferred. Hybrid constructions of various materials are likewiseadvantageous.

The dielectrically hindered discharge is known to be a transient gasdischarge which is made up of filament-like discharges having a shortduration. The distance between the electrodes is generally about onemillimeter. Both electrodes appropriately comprise metal. A dielectric,e.g. glass or ceramic, can be applied to them or introduced betweenthem. If the reactor wall itself forms one of the two electrodes, i.e.is made of a metallic material, the resulting arrangement is referred toas a “discharge hindered on one side”.

Preference is given to a dielectrically hindered discharge having afrequency of from 1 Hz to 100 MHz, particularly preferably from 30 Hz to1 MHz, very particularly preferably from 50 Hz to 4 kHz; in particular,all values in the range from 1 to 100 kHz are also advantageous.

Furthermore, when a PRG operated at a power of more than about one wattis used, the electrodes present are advantageously cooled by means of acooling medium. It is in this case advantageous to choose a coolingmedium which has a boiling point of from about 20 to 70° C. at about 300mbar. Thus, a shell-and-tube heat exchanger can be operated, forexample, using water as cooling medium.

The phase to be treated is advantageously passed through the dischargezone of the reactor at a flow velocity of from 0.01 to 100 m/s, inparticular from about 0.1 to 10 m/s. The exposure time per discharge ispreferably from 10 ns to 1 s, i.e. the phase to be treated is preferablypresent in the discharge zone for a total period of from 1 ms to 10minutes at STP, particularly preferably from 100 ms to 10 s at STP, inparticular 1.1 s at 300 mbar abs.

According to the invention, the treatment of the phase is appropriatelycarried out at a pressure of from 0.1 mbar to 10 bar abs., preferablyfrom 1 mbar to 2 bar abs., particularly preferably from 100 mbar to 1.5bar abs., very particularly preferably from 200 mbar to 1 bar abs., inparticular from 250 to 500 mbar abs., with the temperature of the phaseto be treated preferably being set to from 0 to 200° C., particularlypreferably from 10 to 80° C., very particularly preferably from 20 to60° C., in the case of silicon tetrachloride. In the case of germaniumtetrachloride, the corresponding temperature can advantageously also behigher.

Furthermore, nitrogen or another buffer gas which is inert in respect ofthe purification task, preferably argon, or else helium, xenon oranother noble gas or a mixture thereof, can be added to the phase to betreated at one or more points in the process of the invention. Inparticular, such a gas can advantageously be used for regulating thepressure in the PRG.

In addition, a selected halogen provider, for example chlorine, can alsobe added in the process of the invention.

In the process of the invention, the phase to be treated can be treatedone or more times by means of the dielectrically hindered discharge. Theresidence time of the gaseous silicon or germanium tetrachloride in thePRG can thus be set in a targeted manner in order to be able to carryout the treatment according to the invention particularly effectively inone cycle or over a plurality of cycles (circulation mode), i.e., forexample, two, three or more passages around the circuit.

However, continuous operation in a single pass is generally preferred.In this case, it is advantageous to use apparatuses which allow asufficient residence time, for example plants in which a plurality ofPRGs are connected in series and/or in parallel.

Furthermore, the operation of the process of the invention, particularlyin the case of continuous operation, can advantageously be accompaniedby analytical measurements in the liquid silicon or germaniumtetrachloride fraction, using, for example, the content ofmethyltrichlorosilane (MTCS) as guide parameter. Here, it is possible,for example but not exclusively, to use the CH, CH₂ or CH₃ bandadvantageously for continuous monitoring by means of IR spectroscopy.

In the process of the invention, the phase which has been treated inthis way is generally cooled in stages and the purified SiCl₄ or GeCl₄fraction is discharged, i.e. the pure product is preferably separatedfrom the treated phase by means of a fractional distillation.

In general, the process of the invention is carried out as follows: thephase to be treated is converted into the gas phase, an inert gas and/orchlorine are/is added if desired, the gas phase is subjected to adielectrically hindered discharge in a pressure-rated, heatable and/orcoolable PRG (4), the treatment is monitored by means of a guideparameter and a fraction consisting of high-purity silicon tetrachlorideor germanium tetrachloride is continuously taken off from the treatedphase by means of fractional distillation.

The treatment according to the invention of SiCl₄ or GeCl₄ contaminatedby hydrogen compounds can be carried out in various ways:

DHD treatment of the phase to be purified, i.e. without any furtheraddition.

DHD treatment in the presence of one or more additives such as hydrogenhalide (HX) and/or halogen (X₂) (preferably with X=CO and/or noble gases(He, Ar, Xe) or nitrogen.

DHD treatment firstly without additives and then continuation of thetreatment in the presence of at least one of the abovementionedadditives.

The process of the invention can be carried out particularlyadvantageously without the addition of a reducing agent.

FIG. 1 shows a preferred embodiment of a plant for carrying out theprocess of the invention.

Here, the tetrahalide-containing phase (A) to be purified is taken fromstorage vessel (A) and fed continuously by means of transport unit (2)and vaporizer (3) to the PRG (4) and there subjected to treatment bymeans of a cold plasma. The starting phase (A) advantageously flows intothe discharge or reaction region in the reactor from below, i.e. fromthe unit (4.1). The SiCl₄ phase which has been treated in this way cansubsequently be condensed in a condenser (5) and fed via transport unit(6) to an intermediate vessel (7). In the condenser (5), hydrogenchloride gas (B) is generally separated off from the condensate.Furthermore, product from the intermediate vessel (7) can be fedcontinuously by means of unit (8) into the upper part of atemperature-controlled column (9), with low boilers (C) being dischargedvia the unit (10) and liquid phase from the column (9) being fed bymeans of transport unit (11) to the likewise temperature-controlledcolumn (12) for separating off high boilers (D). Gaseous product fromcolumn (12) can be condensed continuously in the unit (13) and beconveyed as high-purity product phase by means of unit (14) to thereceiver or storage container for product (15).

The decrease in the content of methyltrichlorosilane (MTCS) ormethyltrichlorogermane (MTCGe), which can generally be present in anamount of from 1 to 500 ppm by weight in a silicon or germaniumtetrachloride to be purified, is preferably used as parameter fordetermining the effectiveness of the process of the invention. Thus, forexample, starting from 133 ppm by weight of MTCS, themethyltrichlorosilane is generally no longer detectable after completionof the DHD treatment even without addition of one of the additionalsubstances mentioned, i.e. its value has been able to be reduced to <1ppm by weight (detection limit for the FTIR method) or <0.2 ppm byweight (detection limit for the 1H-NMR method).

An SiCl₄ phase which has been treated in this way and preferably has anMTCS value as guide parameter of about <1 ppm by weight can then bepassed to a separation. The separation can advantageously be effected bymeans of a fractional distillation, preferably giving high-puritysilicon tetrachloride as purified product.

Furthermore, the process of the invention and the apparatus of theinvention have an extremely high effectiveness. Thus, silicon orgermanium tetrachloride containing methyltrichlorosilane (MTCS) ormethyltrichlorogermane (MTCGe) in amounts into the percentage range canbe freed completely of this by means of the DHD treatment process of theinvention. If trichlorosilane (TCS) or trichlorogermane (TCGe) isadditionally present in the SiCl₄ or GeCl₄ phase to be purified, thiscan advantageously be removed at the same time.

The present invention is illustrated by the following example withoutthe subject matter claimed being restricted thereby.

EXAMPLE

In a plant as shown in FIG. 1, the PRG was supplied with 400 kg/h ofSiCl₄ (contaminated by 10 ppm by weight of methyltrichlorosilane) andthe gas phase was treated by means of a cold plasma. The PRG wasequipped with 1 200 micro units, cf. FIG. 2, with the tube length of thedielectrics being 1.5 m and the respective internal diameter being 10mm. The gap was 0.5 mm. The PRG was operated at about 30° C. Thisresulted in a mean residence time of the gas of 1 s in the reactor at apressure of about 300 mbar abs. This corresponded to a residence time atSTP of about 3 s. The treated gas phase was subsequently fractionallycondensed. No methyltrichlorosilane could be detected in the purifiedSiCl₄ product phase obtained in this way.

1. A plasma reactor, comprising: a reactor housing, a high-voltagesupply, and at least one micro unit for the plasma treatment whichconsists essentially of a grounded, metallic heat exchanger, adielectric , a perforated plate, a grid or a mesh and a high-voltageelectrode; wherein longitudinal axes of the dielectric, of thehigh-voltage electrode and of the grounded, metallic heat exchanger areoriented parallel to one another and at the same time parallel to thedirection vector of the force of gravity and the dielectric ismechanically self- stabilized.
 2. A plant, comprising the plasma reactorof claim 1, wherein the plant is for preparation of high-purity silicontetrachloride or high-purity germanium tetrachloride.
 3. The plant asclaimed in claim 2, wherein the dielectric is a tubular dielectricoriented on a perforated plate, the grid or the mesh, and is surroundedby the metallic heat exchanger and a rod electrode or the mesh electrodeprojects completely or partly into the tubular dielectric.
 4. The plantas claimed in claim 2, wherein the dielectric is a tube having a wallthickness from 0.1 to 10 mm, an internal diameter from 1 to 300 mm, anda length from 10 to 3000 mm and comprises quartz glass, Duran glass,borosilicate glass or aluminum oxide.
 5. The plant as claimed in claim2, wherein the electrode comprises a metal or a metal alloy and iscapable of being cooled.
 6. The plant as claimed in claim 2, wherein adistance of respective shortest open spacings between the dielectric andthe high-voltage electrode and between the dielectric and the heatexchanger are identical.
 7. The plant as claimed in claim 6, wherein thedistance of the spacings is from 0.01 to 100 mm.
 8. The plant as claimedin claim 2, wherein the perforated plate, the grid or the mesh has afree cross-sectional area of from 10% to 90%.
 9. The plant as claimed inclaim 2, wherein the heat exchanger is capable of being heated andcooled, and is configured as a shell-and-tube heat exchanger.
 10. Theplant as claimed in claim 2, wherein the plasma reactor comprises from 1to 50,000 micro units.